The present invention relates generally to the manufacture of circuit boards and pertains, more specifically, to the placement of pads of solder paste in a prescribed pattern on a printed circuit board being prepared for the surface mounting of components onto the circuit board by subsequent soldering.
Current manufacturing techniques for making printed circuit boards call for the mounting of components to the circuit board by “surface mounting”. That is, rather than passing the leads of a component through holes in a circuit board and then soldering the leads at the holes, the circuit board is provided with pads of solder paste and the components are placed on the pads for soldering directly to the surface of the board.
In order to place the pads of paste in appropriate locations on the circuit board, just before affixing the components to the board, a stencil-like screen or foil is juxtaposed with the circuit board and provides a pattern of openings registered with the locations where the pads are to be placed. The paste then is spread over the screen with a squeegee to lay down the pattern of pads. The stencil screen is in the form of a thin foil, which is supported by a stretcher frame in an appropriately tensioned state. The frame fits into a machine, which registers the tensioned screen with the circuit board and then applies the paste. Typically, the machine also stretches the stencil foil to achieve the desired tension of the stencil to ensure accuracy during printing.
However, the foregoing stencils pose serious safety and operational problems and are difficult to securely mount to a stretcher apparatus. For example, traditional foil stencils are potential safety hazards to those technicians that handle them due to the presence of sharp edges around the entire periphery of the foil stencil. Such sharp edges can easily cut a person handling the foil stencil and putting them at risk of exposure to infection. This is a particular concern due to the presence of solder paste, which can be toxic, in the work environment. In light of the aforementioned safety hazards, transportation and storage of foil stencils are problematic because technicians are constantly transporting foil stencils to and from a storage location during the normal course of circuit board manufacture. During this normal handling, the foil stencil undergoes bending, for example, that threatens its integrity. As a result, foil stencils are prone to damage.
There have been many attempts in the prior art to address the foregoing problems with foil stencils relating to safety to the handler and damage during handling and storage. For example, it is common in the prior art to mount foil stencils to an aluminum frame with a polyester border to ensure proper tensioning of the foil stencil. While the encapsulation of the periphery of the foil stencil with polyester keeps the foil tight and covers the sharp edges from the handler, it is time consuming and expensive to encapsulate the foil stencil and requires special molding machinery. As a result, such polyester encapsulated foil stencils require an additional preparation step, which typically requires outsourcing to a special subcontractor for such operation. Moreover, these encapsulated foil stencils are difficult to store and cannot be easily retrieved when stacked.
FIG. 1 shows a prior art stencil foil assembly 10 that has a frame 12 on which a stencil foil 14 can be supported. Mounting apertures 16 on the stencil foil 14 are spaced apart and arranged to engage an array of studs 18 that extend upwardly on the frame 12 when the stencil foil 14 is placed on the frame 12. The frame 12 engages a stretching machine (not shown), so that stretching of the frame 12 pulls the stencil foil 14 into tension. A stencil pattern 20 is shown towards the center of the stencil foil of FIG. 1.
In view of the foregoing, there is a demand for a frame for a foil stencil assembly that protects that handler from the sharp edges to avoid injury. There is a further demand for a foil stencil assembly that can be easily stored and retrieved. There is a demand for a foil stencil assembly that can lay flat or hang from a vertical hook in a high density storage cabinet. There is a further demand for a foil stencil assembly that is inexpensive and easy to assemble without special tools or special equipment. There is also a demand for a foil stencil assembly that has stencil tension that does not decrease over time and is machine washable. In addition, there is a further demand for a foil stencil assembly with a large print area.
Also, in the prior art, there is need to facilitate adapting a stencil foil for mounting thereof to a stretcher frame. It is known in the art to bend the peripheral edges of the stencil foil so such periphery can be gripped and secured by a stretching apparatus. However, this requires that a custom stencil frame be created to provide such bent free edges. Moreover, these bent edges are difficult to form and can easily break.
Therefore, there is a need for a stencil foil assembly that can be easily affixed to existing and standard stencil foils to provide a rigid frame for securing the entire assembly into a stretching apparatus for printing.